The correct answer is the following.
The plan of Friar is foreshadowed from the beginning of the play in that people, the audience, hears what he has to say about the use of plants, how to mix them to create toxic formulas or dangerous combinations. The Friar’s plan needs to be executed on time. Not before, not after, because it has its reason. Juliette needs to be dead just for any specific time during the story. Romeo needs to know that Juliette is not really dead, but part of the plant, otherwise that could end in a tragedy.